Xojo doesnt use clang at all
They’ve implemented their own custom front end for LLVM
It’s uh … interesting - lets say that 
No. I quoted that nobody will be able to develop such IDE for all platforms. And that I described. We have no chance to develop an IDE for a language we have no Compiler. That would be the first thing we need.
You’re right - I meant LLVM, but couldn’t remember the term - clang was close enough for me
Ah yes - if it was a “make a new Xojo” then there are a ton of things required as you say
If it was “make a NEW BETTER IDE for Xojo and leverage the existing compiler etc” that might be plausible but the integration between IDE and compiler and debugger would be much the same as it is now in Xojo’s IDE 
As nobody can reach the compiler as command line compiler there is no chance without the Xojo IDE. That said there is no way to build a Xojo IDE with all the needed features like Debugging, Building and so on. What you can develop is an editor for Xojo Programs not more not less.
A Xojo like Ice is not the point to build. It is possible, take a compiler which can leverage LLVM for all platforms and then start to build up the IDE and the platforms and the GUI System. Possible. At least I wouldn’t say impossible. But you will have the same amount of problems Xojo had within years.
VisualFBEEditor
Thanks it never crossed my mind to look at any open source ready to use solutions. And use one of them as a base for building an IDE for Xero Coder SDK. Now finally I will be able to get started with building the IDE as it is open source project and I can fork it on GitHub.
I have to thanks @bgrommes for this. He referred to FreeBasic in another thread.
https://ifnotnil.com/t/what-are-some-of-the-most-important-points-you-learned-over-the-years-as-a-programmer/3009/39